This invention relates generally to hard disk drives of the type generally used with microcomputers for storing digital data and more particularly to improvements therein for automatically retracting the drive read/write heads on power turn off or in the event of an emergency condition, such as loss of electric power.
Hard or rigid disk drives (frequently referred to as "Winchester drives") are used extensively in computer systems for storing digital data. Such drives are characterized by the use of one or more rigid disks or platters mounted on a common spindle for rotation relative to a positioner assembly. The positioner assembly typically comprises a carriage carrying a comb-like structure having a plurality of fingers projecting toward said spindle. Each finger carries a read/write head adjacent to a disk surface for writing data thereon or reading data therefrom. The carriage is typically mounted for substantially radial movement relative to the spindle to position the heads adjacent to a selected disk surface track.
In modern disk drives, the heads are operated very close to the disk surfaces to achieve satisfactorily high recording densities; e.g. approximately 12 thousand bits per inch and one thousand tracks per inch. To accomplish this, a typical head is mounted so as to "fly" very close to the disk surface on a thin layer of air, acting as a bearing, produced as a consequence of the disk surface motion. The head mounting means, which usually includes means for loading or biasing the head toward the disk surface against the air bearing, depends upon the disk surface motion to keep the head flying. When the spindle speed is lower than that necessary to sustain the air bearing, (typically, about 2/3 normal spindle speed), the head no longer flies and thus comes into direct contact with, and drags along, the disk surface. This dragging which occurs both during spindle start-up and spindle turn-off whenever the spindle speed is less than the minimum flying speed, can permanently damage the disk surface.
In order to minimize the effect of disk surface damage, it is prudent to design the disk drive so that the head will always land in a nondata zone (landing zone) of the disk surface. It is additionally prudent to design the disk drive to minimize the duration during which the spindle speed is below flying speed. That is, it is preferable to operate the drive so that the spindle is quickly accelerated up to flying speed during start-up and quickly decelerated to rest during turn-off (attributable to either planned or inadvertant loss of electric power).
Virtually all modern disk drives incorporate some means for avoiding head crashes against a data zone portion of the disk surface, when electric power is lost. For example, U.S. Pat. No. 4,371,903 discloses an emergency head retract system characteristic of the prior art for unloading a head driven by a single winding linear motor positioner. More specifically, U.S. Pat. No. 4,371,903 discloses a system which utilizes the kinetic energy of the spindle motor to provide the energy required to unload the heads by switching the spindle motor windings directly to the linear motor positioner coil.